Contents
Types of Adsorption
Depending upon the nature of forces which hold the molecules of the adsorbate on the surface of the adsorbent, the adsorption is classified into two types:
1) Physical adsorption and
2) Chemical adsorption.
Physical Adsorption
When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van der Waals forces, the adsorption is called physical adsorption.
The attractive forces are weak and, therefore, these can be easily overcome either by increasing the temperature or by decreasing the pressure. Physical adsorption can be easily reversed or decreased.
Characteristics of physical adsorption
(i) Low enthalpy of adsorption: The attractive forces between adsorbent and the adsorbate molecules are weak (van der Waals forces) and therefore, molar enthalpy of adsorption is low.
(ii) Reversible nature: The physical adsorption process is generally reversible and therefore, equilibrium is reached rapidly. Thus,
Solid +Gas ⇔ Gas/Solid+ Heat
When we increase the pressure (or decrease the volume), the equilibrium shifts in the forward direction i.e. adsorption increases. Therefore, more of gas is adsorbed when pressure is increased as the volume of the gas decreases. Gas can be removed by decreasing the pressure.
(iii) Effect of temperature: Since adsorption process is exothermic, therefore, physical adsorption occurs readily at low temperature and decreases with increase in temperature according to Le-Chatelier’s principle.
If the temperature is increased, the kinetic energy of the gas molecules increases and they leave the surface of the adsorbent. Therefore, rise in temperature decreases the extent of adsorption.
(iv) Lack of specificity: It is not specific in nature and therefore, all gases are adsorbed on all solids to the same extent. This is because a given surface of an adsorbent does not show any preference for a particular gas as the van der Waals forces are universal.
(v) Nature of adsorbate: The gases which are easily liquefied (i.e., having higher critical temperatures) are adsorbed readily because van der Waals forces are stronger near the critical temperatures.
(vi) State of adsorbate: In physical adsorption, the state of adsorbate is same as in the bulk.
Chemical Adsorption
When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces or chemical bonds the adsorption is called chemical adsorption or chemisorption.
A chemical reaction occurs between the adsorbed molecules and the adsorbent on the surface. This type of adsorption is irreversible.
Characteristics of chemical adsorption
(i) Enthalpy of adsorption
Attractive forces between adsorbent and adsorbate molecules are strong chemical bonds and therefore, molar heat of adsorption is high.
(ii) Irreversible nature
It is irreversible because chemisorption involves compound formation.
(iii) Effect of temperature
Chemical adsorption first increases with, increase in temperature upto a certain extent and then decreases regularly. A gas adsorbed at low temperature by physical adsorption may change into chemisorption at a high temperature.
(iv) High selectivity
Chemical adsorption involves the formation of chemical bonds between the adsorbed molecules and the surface of adsorbent. Therefore, it is highly selective. Chemical adsorption depends upon the nature of the chemical properties of the gas and the adsorbent. For example: oxygen is adsorbed on metals by virtue of oxide formation and hydrogen is adsorbed by transition metals due to hydride formation.
(v) State of adsorbed species
Since chemical reaction takes place in this type of adsorption, therefore, the state of adsorbed molecules may be different from that in the bulk.
Enthalpy of Adsorption
The amount of heat evolved when one mole of an adsorbate (gas or liquid) is adsorbed on the surface of an adsorbent is called enthalpy of adsorption. The enthalpy of adsorption for chemisorption is larger than that for physical adsorption.
| Physical adsorption | Chemical adsorption |
| The forces between the adsorbate molecules and the adsorbent are weak van der Waals forces. | The forces between the adsorbate molecules and the adsorbent are strong chemical forces similar to chemical bonds. |
| Low enthalpy of adsorption of the order of 20 to 40 kJ mol-1 | High enthalpy of adsorption of the order of 80 to 240 kJ mol-1 |
| Usually occurs at low temperature and decreases with increasing temperature. | It occurs at high temperature and increases with the increase of temperature. |
| It is reversible in nature. | It is irreversible. |
| The extent of adsorption depends upon the ease of liquefication of the gas. More easily liquefiable gases are adsorbed readily. | There is no correlation between extent of adsorption and the ease of liquefication of gas. It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption. |
| It is not specific in nature i.e., all gases are adsorbed on the surface of a solid to some extent. | It is highly specific in nature and occurs only when there is bond formation between adsorbent and adsorbate molecules. |
| No appreciable activation energy is needed. | High activation energy is sometimes needed. |
| The state of adsorbate is same as in the bulk. | State of adsorbate molecules may be different from that in the bulk. |
| It forms multimolecular layers. | It forms mono-molecular layer. |
| Rate of adsorption increases with increase in pressure of adsorbate. | Rate of adsorption usually decreases as the pressure increases. |
| It depends on the surface area. It increases with an increase of surface area. | It also depends on surface area and also increases with an increase of surface area. |
Types of Forces
The atoms or molecules of a solid surface have unbalanced or residual attractive forces. As a result, the surface of the solid has the tendency to attract the molecules of a gas or a liquid when they come in its contact. Since energy is released because of attraction, the solid gets a more stable state. The adsorbed atoms or molecules are held on the surface of a solid by physical van der Waals forces or chemical forces due to residual valence bonds.
The forces of attraction increase when a solid is broken into pieces or crushed to powder. This is because of formation of more surfaces.As a result, the tendency for adsorption becomes large.